Ross Valley Watershed

Watershed Overview

The 28-square mile Corte Madera watershed extends from Mt. Tamalpais and White’s Hill through the communities of Fairfax, Sleepy Hollow, San Anselmo, Ross, Kentfield, Greenbrae, Larkspur, and Corte Madera to San Francisco Bay. The watershed includes 44 miles of stream channels. Ross Creek drains the northern slope of Mt. Tamalpais; San Anselmo Creek and its tributaries drain the northwestern portion of the watershed. The two channels join to form Corte Madera Creek, which continues through more than a mile of concrete-lined channel past the confluences of Larkspur and Tamalpais Creeks and into the salt marsh at the mouth.

Despite the dense development in the valley floors, the Corte Madera watershed supports a great diversity of habitats from redwood forest to serpentine outcrops, chaparral, oak woodlands, grasslands, and tidal wetlands. It is home to many protected species including at least 17 plants, steelhead trout, spotted owls, San Pablo Song Sparrow, California clapper and black rails, and salt marsh harvest mouse. Corte Madera Marsh Ecological Reserve is recognized as an Important Bird Area by the Audubon Society.

In 2006, the County of Marin, City of Larkspur and Towns of Fairfax, Ross and San Anselmo formed the Ross Valley Flood Protection and Watershed Program (RVFPWP) to develop and implement a flood management program that promotes with healthy watershed processes.

Watershed History

Human settlement

The Miwok lived for thousands of years in the Corte Madera watershed, gathering pinole and acorns, hunting, and salmon-fishing. The watershed retains many traces of their habitation including seven mounds in what is now the Town of Ross.

In the early post-Miwok era, the northwest and southeast portions were divided into two separate land grants. The Canada de Herrera, a 6,658-acre rancho that includes the areas that are now Fairfax, Sleepy Hollow, and part of San Anselmo, was granted to Domingo Sais in 1839. His family used the land for crops, sheep, horses, and cattle and fished San Anselmo Creek for salmon. The Punta de Quintin, Corte de Madera, La Laguna y Canada de San Anselmo, an 8,887-acre rancho established in 1840, is now the towns of San Anselmo, Ross, Kentfield, and Larkspur. The rancho’s owner, Juan Cooper, harvested timber from the property and was also granted license to hunt otter at the mouth of Corte Madera Creek. In 1857, James Ross bought much of the Rancho, continued logging and also started a regular schooner route to San Francisco to transport the wood. His family established an estate at the site that is now the Marin Art and Garden Center.

In the late 1800s, Ross Valley was a fertile farming area with orchards, vineyards, poultry, and dairies. The last big farm, the 1,600 acre Sleepy Hollow Dairy Ranch, was sold to developers in 1925.

Development in the watershed increased substantially when the North Pacific Railroad Coast Railroad built two lines, an 1874 spur from San Anselmo to San Rafael and an 1875 line from Sausalito, through San Anselmo, to Tomales and north to Sonoma County . The railroad was originally used to ship timber to San Francisco via the Sausalito Ferry, but as stops on the line became more accessible, people moved in and land values rose. The 1906 earthquake also caused an increase in the population in the watershed; the largest and continuing surge started in 1937 with the construction of the Golden Gate Bridge.

Watershed Today

Changes to Watershed Processes

Corte Madera Creek and its tributaries responded to the intensive timber harvesting and livestock grazing of the 1800s by incising into the Holocene valley fill. Evidence from riparian tree age and rooting location confirms that channel entrenchment began more than 100 years ago, with the process progressing quickly after the initial disturbances and leveling off in the early and mid 1900s. Large trees growing within the channel banks are approximately 50 years old and were likely established immediately after the 1955 flood, which formed high gravel bars. Although the channels are still responding to the 1800s land use and subsequent urbanization the effects are slowing and less dramatic. As stated by Stetson (2000), ongoing channel responses include:

headward advance of 1st order tributaries,

reduced bed incision and bank erosion in the upper alluvial channel network, and

slowing of channel aggradation in the lower reaches of the watershed.

Exposed bedrock outcrops and constructed grade-control structures throughout the channel network have slowed channel incision while accelerating channel widening. Nearly 50% of the banks have been stabilized with rock or concrete to protect landowners from property loss through bank retreat (Stetson 2000 citing Friends of Corte Madera Creek Watershed 1997). Channel widening is a natural process following rapid channel incision, and allows the development of inset floodplains that are important for habitat value and increased flood capacity. Geomorphic recovery processes are ongoing, with inset floodplains occurring in areas where the channel was not restricted from widening and pool/riffle sequences forming in the stable bed. Dense urbanization up to the top of streambanks and unnaturally narrow channels restrict instream habitat recovery and limit channel capacity.

The tidal reaches of the system are heavily impacted and have been modified for flood management. In the 1960s, the Army Corps of Engineers designed and constructed an earthen trapezoidal channel on the lower 4.5 miles of creek through the towns of Corte Madera, Larkspur, Kentfield, and Ross. Lower Corte Madera Creek has been widened and straightened. These lower reaches are sediment aggradation and storage zones for upland and tidally-derived sediment.

Sediment is delivered to the channels from upland sources such as gully development, overland flow, and landslides, as well as from channel bed and bank erosion. It is estimated that the latter accounts for only 9% of the annual bedload transported in the system, while the upland sources account for 91% (Stetson 2000 1). Together the San Anselmo Creek and Sleepy Hollow Creek subwatersheds generate 55% of the total annual bedload, while Ross Creek and Fairfax Creek subwatersheds only generate about 10% of the bedload each. These differences are due to variations in geology, topography, vegetation types, and land use.

Habitat Types

Vegetation

The Corte Madera Creek watershed originates in the foothills of Mount Tamalpais and extends through steep and narrow valley floors before entering San Francisco Bay. The watershed supports a diversity of communities from hilly headwaters to saltwatermarshes along the bay. The upper stream reaches support relatively natural stream channels and the lower and middle reaches support urban creek habitat. Near the confluence with Ross Creek, Corte Madera Creek is confined to a concrete-lined channel for approximately one mile. The upper reaches of the watershed are owned by Marin County Open Space District and Marin Municipal Water District. The watershed supports both rearing and spawning habitat for steelhead trout.

Historically, the watershed supported extensive native forests and grasslands, healthy riparian systems, and wetlands in the lower watershed. Currently, the upper watershed is composed primarily of oak-bay woodland, mixed evergreen forest, including coast redwood and Douglas fir, grassland, chaparral, northern coastal scrub, and serpentine outcroppings. Urban areas dominate the middle and lower reaches of the watershed. Developed urban uses often abut the creek bank. Commercial buildings, fences, and backyards of residences are close to the creek bank in some areas. Along the lower tidal reach, residential and commercial uses abut the Corte Madera Channel. Marin County Open Space District and Marin Municipal Water District lands provide habitat connectivity with adjoining watersheds.

Due to its steep topography, tidal marshes have always been limited along the bayshore (Goals Project 1999 2). Today, only a few tidal marshes remain in the watershed. The largest is the 1000-acre Corte Madera Marsh Ecological Preserve. This preserve is recognized as an Important Bird Area (IBA) and is known to support California clapper rail, a few black rail, and a large population of San Pablo song sparrow. The mudflats at the preserve are also used extensively by migrating shorebirds (Audubon 2008). Just south of the marsh, there is a tidal channel, San Clement Creek. Harbor seals haul-out in the vicinity of the channel (Marshall, et a., 1993 3).

The Friends of Corte Madera Creek maintain a detailed website outlining biological resources and restoration efforts within the watershed (www.friendsofcortemaderacreek.org). The Marin Audubon Society website also provides information of many of the county’s birds and the preserves owned by the Society within the watershed (www.marinaudubon.org).

Fish and Wildlife

The watershed supports a number of special-status plants and animals. Northern spotted owl territories occur in wooded habitats along several creeks. Noteworthy species in the lower watershed include salt marsh harvest mouse, California clapper rail, California black rail, Point Reyes bird’s-beak, and marsh microseris.

Introduced species include rainwater killifish, western mosquitofish, black crappie, and common carp (Leidy 2007). Detailed fisheries information and habitat descriptions are provided in the summary of surveys section below (Rich 2000 and 2006).

Within the tidal reaches of Corte Madera Creek, bay goby, speckled sanddab, northern anchovy, striped bass, staghorn sculpin, plainfin midshipman, shiner surfperch, bay pipefish, and longjaw mudsucker have been documented. In 1991, the first bigmouth sole captured in the San Francisco Bay was netted in the tidal area of Corte Madera Creek (Marshall, et al., 1993).

There are no reported occurrences of California red-legged frog, a federally-listed as threatened and California Species of Special Concern species, foothill yellow-legged frog or northwestern pond turtle, both California Species of Special Concern species, within the watershed (CDFG 2008 6)

Common herpetofauna have been documented in the watershed (Friends of Corte Madera Creek Watershed 1997 7). These include California newt, Pacific giant salamander, arboreal salamander, California slender salamander, yellow-eyed salamander (Ensatina), Pacific tree frog, and western fence lizard. Turtles have been observed at Phoenix Lake but they have not been identified to species. The watershed also supports a number of snake species.

From 2005 through 2007, an estuary-wide study of the California clapper rail population was conducted by PRBO Conservation Science and Avocet Research Associates (Herzog, et al., 2005 8; Liu, et al., 2008 9). Rails were detected in suitable habitat within the watershed; noteworthy locations, where a number of rails were detected, included the Corte Madera Marsh Ecological Preserve.

Human Habitation and Land Use

Land Use Imperviousness

Most of the watershed is developed; a portion is protected by the Marin County Opens Space District, primarily along the ridgelines. In addition, the Marin Municipal Water District has watershed lands located in the southern and western portions. The remainder is in residential use and related services. Some of the development has taken place along the floodplain of Corte Madera and San Anselmo Creek and in areas that have been filled along the San Francisco Bay.

There are five incorporated municipalities: Fairfax, San Anselmo, Ross, Larkspur, and Corte Madera. The watershed also has several unincorporated communities including Greenbrae, Kentfield, and Sleepy Hollow. Each of the cities and communities, with the exception of Sleepy Hollow, has downtown shopping districts; Corte Madera has two substantial shopping areas next to Highway 101. The watershed has an unusually high amount of institutional land, with San Quentin federal prison on point San Quentin and the College of Marin in Kentfield.

Less than 10% of the watershed remains available for development. Most future development is likely to be from infill and redevelopment. County growth projections show an increase in the number of households, but a small decrease in the total population—likely resulting from demographic changes.